1. Field of the Invention
This invention relates to communications and, more particularly, to a rate control system for a wireless communications system.
2. Description of Related Art
Wireless communications systems include conventional cellular communication systems which comprise a number of cell sites or base stations, geographically distributed to support transmission and receipt of communication signals to and from wireless units that may actually be stationary or fixed. Each cell site handles communications over a particular region called a cell, and the overall coverage area for the cellular communication system is defined by the union of cells for all of the cell sites, where the coverage areas for nearby cell sites overlap to some degree to ensure (if possible) contiguous communications coverage within the outer boundaries of the system's coverage area.
When active, a wireless unit receives signals from at least one base station or cell site over a forward link or downlink and transmits signals to (at least) one cell site or base station over a reverse link or uplink. There are many different schemes for defining wireless channels for a cellular communication system, including TDMA (time-division multiple access), FDMA (frequency-division multiple access), and CDMA (code-division multiple access) schemes. In CDMA communications, different wireless channels are distinguished by different codes or sequences that are used to encode different information streams, which may then be modulated at one or more different carrier frequencies for simultaneous transmission. A receiver can recover a particular stream from a received signal using the appropriate code or sequence to decode the received signal.
Due to the delay-intolerant nature of voice communication, wireless units in conventional cellular systems transmit and receive over dedicated channels between a wireless unit and a base station. Generally, each active wireless unit requires the assignment of a dedicated link on the forward link and a dedicated link on the reverse link. Current wireless communications systems are evolving which provide access to packet data networks, such as the Internet, and support a variety of data services. For example, support for multimedia applications (voice, video and data) is important for any network connected to the Internet. These applications have specific requirements in terms of delay and bandwidth. Traditional data applications are typically bursty and, unlike voice communications, relatively delay tolerant. As such, using dedicated links to transmit data is an inefficient use of network resources. Consequently, resource allocation systems have been devised to make more efficient use of network resources using different quality of service (QoS) classes for the different types of traffic based on the delay-tolerant nature of the traffic.
The Universal Mobile Telecommunications System (UMTS) was designed to offer more wireless link bandwidth and QoS features. FIG. 1 shows a typical UMTS network 10 which can be divided into a radio access network (RAN) 12 and a core network (CN) 14. The RAN 12 comprises the equipment used to support wireless interfaces 16a–b between a wireless unit 18a–b and the UMTS network 10. The RAN 12 includes NodeBs or base stations 20a–c connected over links (Iub links) 21a–c to radio network or base station controllers (RNC) 22a–b. The interface between the base station and the RNC is referred to as the Iub interface or link, and the interface between two RNCs is referred to as the Iur interface although UMTS Release 99 does not require Iur to support routing. Currently, both the Iub and Iur interfaces are based on ATM, and ATM switches are allowed between NodeBs and RNCs in the UMTS architecture.
The core network 14 comprises the network elements that support circuit based communications as well as packet-based communications. In establishing a circuit channel to handle circuit-based communications between the wireless unit 18b and a public switched telephone network (PSTN) 24 or another wireless unit, the base station 20b receives (in the uplink) and transmits (in the downlink), the coded information (circuit voice or circuit switched data) over the wireless interface or link 16b. The RNC 22b is responsible for frame selection, encryption and handling of access network mobility. The RNC 22b forwards the circuit voice and circuit switched data over a network, such as an asynchronous transfer mode (ATM)/Internet Protocol (IP) network to a 3G mobile switching center (MSC) 30. The 3G-MSC 30 is responsible for call processing and macromobility on the MSC level. The 3G-MSC 30 establishes the connectivity between the wireless unit 18b and the PSTN 24.
In establishing a packet channel to handle packet-based communications between the wireless unit 18a and a packet data network (PDN) 34, such as the Internet, the base station 20a receives (in the uplink) and transmits (in the downlink), the coded information over the wireless interface or link 16a. In the uplink direction, the RNC 22a reassembles the packets as sent by the wireless unit 18 and forwards them to SGSN 40. In the downlink direction, the RNC 22a receives the packets and segments them into the right size transport frames or blocks to be transferred across the wireless link 16a. The SGSN 40 provides packet data session processing and macromobility support in the UMTS network 10. The SGSN 40 establishes connectivity between the wireless unit 18a and the PDN 34. A GGSN 42 is the gateway to external PDNs. The GGSN 42 acts upon requests from the SGSN 40 for packet data protocol (PDP) session establishment.
On the downlink in a current system, after data is generated and arrives at the RNC 22a from the network, it is mapped to Iub frames before being sent on the Iub-link 21a. For example, once the TTI (Transmission Time Interval) and the Radio Bearer Rate (ranging from 64 Kbps to 384 Kbps for web-browsing interactive service) are given, the mapping can be determined by looking up the corresponding parameters, such as the transport format set shown in Table 1 below. In a current UMTS system, each incoming packet is mapped to the least possible number of frames or TTIs. In this example, after the medium access control (MAC) maps the incoming packet into frames and adds the appropriate headers, the frames are passed onto the dedicated channel framing protocol (DCHFP) layer. The DCHFP layer adds framing protocol headers to the frames to get the Iub frames. The Framing Protocol—Protocol Data Unit (FP PDU) is then passed to the ATM adaptation layer (AAL2) layer described in the standard identified as ITU-T I.363.2. FP PDUs from different terminals are appended with the AAL2 header that identifies the particular wireless unit or user to which the frames belong. These AAL2 layer frames are then packed into ATM cells before being transmitted on the Iub link. The link from RNC to NodeB can have a variety of different bandwidths, such as T1 (1536 Kbps) or E1 (1920 Kbps).
Accordingly, at the RNC, data is received for each UE, and the user data packets go through various layers before actually being transmitted on the Iub link. The incoming data packets are fragmented and appropriate overheads are added before being sent on the Iub link depending on the type of data service being provided. Each packet data service belongs to one of the 4 QoS classes as specified in the 3GPP standards: conversational, interactive, streaming and background. Table 1 shows a typical parameter set for the transport channel for the interactive class. In this example, the transport format set (TFS) contains 5 different transport formats or sizes (TF0–TF4). As shown in Table 1, if transport format TF1 is selected, one 42 byte transport block is sent during the TTI (for example, 20 ms.). If the largest transport format TF4 is selected for sending data over the duration of a TTI, then four (4) 42 byte transport blocks are sent during the TTI, giving the user the peak rate of 64 Kbps. The TFS will be different based on the type of service, and the Radio Bearer Rate and the TTI. In this example, the MAC layer processes the user data according to the specified TFS. The transport format can be referred to as transport format block (TFB) or transport block set size.
TABLE 1Transport channel parameters for Interactiveor background/64 Kbps PS RABRLCLogical channel typeDTCHRLC modeAMPayload sizes, byte40Max data rate, kbps64RLC header, byte 2MACMAC header, byte 0MAC multiplexingN/ALayer 1TrCH typeDCHTB sizes, bytes42TFSTF0, bytes0 × 42TF1, bytes1 × 42TF2, bytes2 × 42TF3, bytes3 × 42TF4, bytes4 × 42TTI, ms20Transport Format Set for the Interactive/background service type for the 64 Kbps Radio Bearer.
When data is received by the RNC for a particular wireless unit or user, the MAC layer creates the appropriate TFB and passes it to the next layer. For example, suppose the RLC buffer holds a 1500 byte packet. The MAC would create 9 TF4's (each with size 160 bytes) which would hold 1440 bytes of user data. The remaining 60 bytes would be placed in the smallest TFB that would hold it, namely TF2 (size 80 bytes).
Due to soft handoff in CDMA systems, there is a strict deadline for transmission of frames from the NodeB to the user after the call is established. If a frame arrives at the NodeB after the deadline has passed, the frame is discarded, thus affecting the QoS. In order to provide reasonable bandwidth utilization on the Iub interface or link (defined as the average fraction of time the link is in use), statistical multiplexing of sources is necessary, leading to variable arrival times (jitter) at the NodeB. Due to the statistical multiplexing on the Iub link, there may be temporary periods when the offered traffic (measured as the total offered bit rate across all sources being multiplexed) exceeds the capacity of the Iub link. For example, one potential problem with the mapping of user data to TFBs as described above is that the offered traffic might well exceed the capacity of the Iub link. For example, with overheads, the peak rate of a user is around 82.5 Kbps. For a T1 link (1536 Kbps), the number of users that can be supported at full rate are 1536/82.5=18. If the number of users with non-empty RLC buffers exceeds 18 at any given point in time, then the input rate to the Iub is more than it can handle. Thus excessive frame discards occur at the NodeB, in addition to the inefficient use of the link in terms of link utilization.